This invention relates in general to circuits for controlling fuel injectors for vehicle engines and in particular to a supplemental fuel injector control circuit.
Fuel injection provides carefully controlled metering of fuel supplied to a vehicle engine. The careful control of fuel supply enhances engine performance and mileage while reducing harmful emissions. Referring now to the drawings, there is shown in FIG. 1 a typical known circuit for controlling a fuel injector valve. The circuit includes an Engine Control Unit (ECU) 10 having a voltage output port 11 that provides a voltage V+ connected to a high end 12a of a fuel injector coil 12. The other, or low, end 12b of the injector coil 12 is connected to an ECU injector control port 13. The ECU 10 includes an electronic switch which is shown in FIG. 1 as an injector Field Effect Transistor (FET) 14. While an FET is shown in FIG. 1, the electronic switch could be any other device that is capable of handling the coil current. The injector FET 14 has a drain terminal 16 connected through the injector control port 13 to the coil low end 12b and a source terminal 18 connected through a small current sensing resistor 20 to ground 22. The injector FET 14 also has a gate terminal 24 that is normally connected to ground, causing the FET 14 to be in a non-conducting state and thereby blocking any current flow through the injector coil 12.
When engine conditions require an injection of fuel, a voltage is applied to the injector FET gate terminal 24, causing the FET 14 to switch to a conducting state. As the injector FET conducts, the low end 12b of the injector coil 12 is connected through the current sensing resistor 20 to ground 22 allowing a current ic to flow through the coil that causes the injector to open and supply a charge of fuel to an associated engine cylinder. When fuel is no longer needed, the injector FET gate terminal 24 is returned to ground, switching the FET 14 back to a non-conducting state and interrupting the current ic supplied to the injector coil 12. Upon interruption of the injector coil current, the injector closes, terminating the supply of fuel to the engine cylinder. The voltage Vi across the sensing resistor 16 is proportional to the current passing through the injector coil 12 and is fed back to the ECU 10 for monitoring the operation of the injector. Also, the ECU 10 monitors the drain terminal voltage VD to assure that the injector and the injector FET 14 are operating correctly. When the injector FET 14 is in its non-conducting state with the injector closed, the drain terminal voltage VD is equal to the voltage V+ appearing at the ECU voltage output port 11. When the injector FET 14 is in its conducting state with the injector open, the drain terminal 16 is effectively connected to ground 22 and the drain terminal voltage VD is approximately equal to zero. While one injector coil 12 is shown in FIG. 1, a similar circuit is provided for each of the engine cylinders.
When modifications are made to a vehicle engine, such as replacing the exhaust system, the stock ECU 10 no longer provides the correct fuel amount across the engine's operating range. As most modifications improve air flow, the resulting fuel/air mixture is typically lean in certain conditions. As illustrated in FIG. 2, it is known to connect an Auxiliary Fuel Injector Controller (AIC) 32 to the low end 12b of the fuel injector coil 12 to provide additional fuel to correct the lean condition by extending the time period that the injector is open. Components shown in FIG. 2 that are similar to components shown in FIG. 1 have the same numerical identifiers. The AIC 32 includes a second FET 34 as an electronic switch with a drain terminal 36 connected to the low end 12b of the fuel injector coil 12 and a source terminal 38 connected directly to ground 22. When in its conducting state, the AIC FET 34 provides an alternate path to ground for the injector coil current and thus keeps the injector open. While an FET is shown in FIG. 2, the electronic switch could be any other device that is capable of handling the coil current. During operation, when the injector FET 14 in the ECU 10 is switched to its non-conducting state, a control voltage can be applied to the gate terminal 40 of the second FET 34 in the AIC 32 to keep the injector open.
Although less common, some modifications such as cam replacement, cause a rich condition requiring the AIC to remove fuel under certain conditions by reducing the time period that the injector is open. For example, an AIC FET could be inserted in series with the injector FET 14 to interrupt the current being supplied to the injector coil before the ECU would normally switch the injector FET 14 to its non-conducting state (not shown).
ECUs are becoming increasingly sophisticated, providing advanced diagnostic capabilities to detect problems within the system. Certain ECUs monitor the voltage and/or current to the fuel injector to determine if the injector and injector controller are operating properly by comparing the duration and magnitude of the coil current flow and duration and magnitude of the change in drain voltage to acceptable operating ranges for the parameters. Adding an AIC to these ECUs can cause the actual time duration and/or magnitude of each monitored parameter to fall outside of the acceptable range. For example, when the second FET 34 in the AIC 32 holds the injector open longer to correct lean fuel air mixture, the low end 12b of the injector coil 12 is pulled to ground. Thus, even though the ECU injector FET 14 is not conducting, its drain terminal voltage VD is zero, as if the FET 14 were conducting. As described above, the drain voltage VD is monitored by the ECU 10 which will erroneously conclude that the ECU injector FET 14 is still conducing and will generate an engine error code, indicating a fault in the injection circuit. Accordingly it would be desirable to provide a circuit that would allow operation of the AIC without triggering ECU error messages.